As an alternative to the less than ideal MDF, a cabinetmaker friend of mine recently turned me onto “MDO”, aka “Signboard”. It is made up of phenolic resin coated paper laminated between layers of ply (11 layers of wood in 3/4”) and twice again as many of the phenolic- resin impregnated paper. Its what roadsigns are made of, its waterproof, flat , flat, flat and did I mention waterproof?
You wont find it in Home Depot or Lowes, I have looked. I have had success at many of the local lumbery...